Liquid dispensers have been used in analyzers for the detection of the concentration of liquid analytes using as analysis means, test elements that contain within themselves the necessary reagents to permit such detection. Examples of such analyzers are described in U.S. Pat. Nos. 4,287,155, issued Sept. 1, 1981, and 4,340,390, issued July 20, 1982. Examples of such test elements appear in U.S. Pat. Nos. 3,992,158, issued Nov. 16, 1976; 4,053,381, issued Oct. 11, 1977; and 4,258,001, issued Mar. 24, 1981. The conventional method for dispensing liquid onto such test elements using such analyzers has been to aspirate test liquid from a relatively large container, into a dispensing container. The dispensing container is then moved to a position immediately above such a test element, and a fraction (e.g., 10 .mu.l) of the aspirated liquid is dispensed. The dispensing container is fluidly connected, in such analyzers, to a pressurizing means. Such means generates both the operative partial vacuum needed to aspirate an amount of liquid into the container, and the partial pressure operative to dispense that aspirated liquid, in fractional amounts, onto a plurality of test elements. A pressure transducer is also conventionally included to ascertain the pressure within the container, so as to detect the occurrence of the desired dispensing event versus a failure to dispense. A microprocessor generally is used to control the apparatus in response to the conditions sensed.
Because only a fraction of the liquid is dispensed each time, one prior approach to dispensing liquid onto a test element has been to vent the dispensing container after each dispensing event. Such an approach is described in U.S. Pat. No. 4,041,995, issued by R. L. Columbus on Aug. 16, 1977, and in U.S. Pat. No. 4,452,899 (col. 4, lines 34-40) issued by Wilton Alston on June 5, 1984. Although this approach generally has worked satisfactorily, on occasion the volume of liquid so dispensed has deviated from that desired. This occurs as follows:
As is described in the aforesaid '899 patent, movement of the dispensing probe into position directly above a test element can cause prespotting of the test element when the probe decelerates to a stop, unless the process includes a partial withdrawal of the meniscus in the dispensing aperture. Such partial withdrawal is achieved by backing up the dispensing pump of the pressurizing means enough to create a slight partial vacuum. This in turn requires any vent that is present to be closed, and remain closed during the actual dispensing step. While the vent is closed, evaporation of the liquid can occur, causing a .DELTA.P pressure to build in the air above the liquid in the dispensing container. Such build-up of pressure adds to the pressure delivered during dispensing, so as to cause a larger volume to be dispensed than would be the case if no build-up had occurred, e.g., if venting had occurred immediately before dispensing. The build-up of pressure can also occur when venting cannot be used. An example of this is those instances when a relatively large volume of liquid (e.g., 110-230 .mu.1) is present in the dispensing container. In that case, the vent is kept closed between dispensing events, because the weight of the large volume would cause the liquid to run out of the container if the vent were open. In other words, at large liquid volumes it has been the practice to operate such dispensing apparatus as though no vent were present. Because the vent is closed, a build-up of pressure tends to occur.
Regardless of the reason for the build-up in air pressure, it is relatively unpredictable. Thus it cannot be compensated for on an a priori basis. As a result, significant deviations from the desired volume can occur in an unpredictable fashion, for example, as much as 10%.
The problem then, prior to this invention, has been to provide such a dispensing apparatus that achieves greater uniformity in the amount of liquid that is dispensed.